1. Field of the Invention
The present invention relates to a double-wrap brake band assembly which is used in an automatic transmission, or the like, for a car, and to a technology for facilitating and securing a braking control.
2. Related Background Art
A band brake apparatus is widely used for braking a transmission element or rotation element in an automatic transmission for a car or in a general industrial machine. A band brake apparatus generally comprises a brake band which is formed by attaching a frictional material to the inner peripheral surface of one steel plate in an annular form. This brake band is constricted by an actuator to fasten a transmission element or a rotation element disposed on the inner side thereof to apply the brake. Therefore, recently, in order to reduce the working power of the actuator and to enhance the controllability of the brake, there is proposed a double-wrap band brake assembly which employs a double-wrap brake band.
As illustrated in FIG. 20 and FIG. 21 (a view seen in the direction indicated by the arrow F in FIG. 20), a double-wrap brake band 3 of a double-wrap band brake assembly is mainly composed of an annular middle band 11 and a pair of annular outer bands 15 which are welded to the middle band 11 through a coupling plate 13 in a state that the respective free ends thereof of the outer bands 15 are opposed to the free end of the middle band 11. Frictional materials 17 and 19 are attached to the inner peripheral surfaces of the middle band 11 and the outer bands 15, respectively. While an anchor bracket 21 which is latched by an anchor pin 5 on the side of a main body casing 1 is welded to the working ends of the outer bands 15, an apply bracket 25 for receiving a force in the direction in which the middle band 11 and the outer bands 15 are constricted by means of a push rod (apply pin) 23 of an actuator 7 is welded to the working end of the middle band 11. In this double-wrap brake band 3, when the apply pin 23 of the actuator 7 is operated in the direction indicated by the arrow in FIG. 20, the middle band 11 and the outer bands 15 are constricted so as to brake a transmission element (hereinafter called the drum) 27 which is fitted in the double-wrap brake band 3.
Incidentally, when the conventional double-wrap band brake assembly is used to brake a rotation element or the like of an automatic transmission, it sometimes becomes a hindrance to a smooth gear change operation. For example, in the above-described double-wrap brake band 3, when it is driven by the apply pin 23 and constricted to thereby come into sliding contact with the drum 27, the bands 11 and 15 wrap around the drum 27 therein by the self engagement effect. As a result, as indicated by the broken line in FIG. 4, a value of the dynamic friction coefficient at the beginning of braking momentarily rises so as to apply the brake on the drum 27 rapidly, whereby, as indicated by the broken line in FIG. 5, a ratio of torque transmission reaches 100% in an extremely short period, and a comparatively large gear change shock is brought about. Particularly, in an area with a small throttle angle in which the transmission torque is comparatively small, a correct braking control is required. However, in the double-wrap band brake assembly, a working stroke of the apply pin 23 is large, which results in a deteriorated response performance and a large torque capacity. As a result, it is very difficult to control a servo pressure and a servo working amount. That is, it is desirable that the bands 11 and 15 come into sliding contact with the drum 27 with a predetermined sliding at the beginning of the brake. However, if the servo pressure and the servo working amount fluctuate, the brake is applied with a little or no sliding amount so as to produce a great transmission shock.
On the other hand, the present inventors, et al., attempted to cause practically no self engagement effect described above by positively introducing an automatic transmission oil into between the double-wrap brake band 3 and the drum 27. However, in this case, with advance of constriction of the double-wrap brake band 3 and the removal of the automatic transmission oil, the dynamic friction coefficient between the double-wrap brake band 3 and the drum 27 gradually increases correspondingly, as shown by the two dot chain line in FIG. 4. Thereby, the drum 27 is slowly braked and, as indicated by the broken line in FIG. 5, a comparatively long time is required for obtaining 100% of the torque transmission ratio. As a result, it becomes difficult to detect a brake starting point of the double-wrap band brake assembly, so that driving control of another transmission element of the automatic transmission can not be smoothly effected.